System and method for energy delivery to tissue while monitoring position, lesion depth, and wall motion

ABSTRACT

Systems and methods for ablating tissue include an ablation device having an energy source and a sensor. The energy source provides a beam of energy directable to target tissue, and the sensor senses energy reflected back from the target tissue. The sensor collects various information from the target tissue in order to facilitate adjustment of ablation operating parameters, such as changing power or position of the energy beam. Gap distance between the energy source and target tissue, energy beam incident angle, tissue motion, tissue type, lesion depth, etc. are examples of some of the information that may be collected during the ablation process and used to help control ablation of the tissue.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 13/630,697 (Attorney Docket No. 31760-713.301), filed Sep. 28,2012, which is a continuation of U.S. patent application Ser. No.12/609,759 (Attorney Docket No. 31760-713.201), filed Oct. 30, 2009,which is a non-provisional of, and claims the benefit of U.S.Provisional App. No. 61/109,873 (Attorney Docket No. 31760-713.101),filed Oct. 30, 2008, the entire contents of each are incorporated hereinby reference.

The present application is also related to U.S. Provisional PatentApplication Nos. 61/110,905; 61/115,403; 61/148,809; 61/109,973;61/109,875; 61/109,879; 61/109,881; 61/109,882; 61/109,889; 61/109,893;61/254,997; and U.S. patent application Ser. Nos. 11/747,862;11/747,867; 12/480,929; 12/480,256; 12/483,174; 12/482,640; 12/505,326;12/505,335; the entire contents of which are incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present application generally relates to systems and methods forablating tissue. More specifically, the present application relates tothe treatment of fibrillation or other arrhythmias of the heart by usingultrasound energy, and even more specifically, the present applicationrelates to ablation systems and methods used to treat atrialfibrillation that provide information related to position of theablation device relative to the tissue, as well as providing informationabout depth of the lesion and motion of the energy source relative tothe tissue.

The condition of atrial fibrillation (AF) is characterized by theabnormal (usually very rapid) beating of the left atrium of the heartwhich is out of synch with the normal synchronous movement (‘normalsinus rhythm’) of the heart muscle. In normal sinus rhythm, theelectrical impulses originate in the sino-atrial node (‘SA node’) whichresides in the right atrium. The abnormal beating of the atrial heartmuscle is known as ‘fibrillation’ and is caused by electrical impulsesoriginating instead at points other than the SA node, for example, inthe pulmonary veins (PV).

There are pharmacological treatments for this condition with varyingdegree of success. In addition, there are surgical interventions aimedat removing the aberrant electrical pathways from PV to the left atrium(‘LA’) such as the ‘Cox-Maze III Procedure’. This procedure has beenshown to be 99% effective but requires special surgical skills and istime consuming. Thus, there has been considerable effort to copy theCox-Maze procedure using a less invasive percutaneous catheter-basedapproach. Less invasive treatments have been developed which involve useof some form of energy to ablate (or kill) the tissue surrounding theaberrant focal point where the abnormal signals originate in PV. Themost common methodology is the use of radio-frequency (‘RF’) electricalenergy to heat the muscle tissue and thereby ablate it. The aberrantelectrical impulses are then prevented from traveling from PV to theatrium (achieving the ‘conduction block’) and thus avoiding thefibrillation of the atrial muscle. Other energy sources, such asmicrowave, laser, and ultrasound have been utilized to achieve theconduction block. In addition, techniques such as cryoablation,administration of ethanol, and the like have also been used.

More recent approaches for the treatment of AF involve the use ofultrasound energy. The target tissue of the region surrounding thepulmonary vein is heated with ultrasound energy emitted by one or moreultrasound transducers.

When delivering energy to tissue, in particular when ablating tissuewith ultrasound to treat atrial-fibrillation, a substantially transmurallesion (burning all the way through the tissue) must be made to form aproper conduction block. Achieving a substantially transmural lesionthough has many challenges. For example, the physician must insureproper alignment of the energy-delivering device relative to the targettissue. If the energy source is too far away from the tissue, the energyreaching the tissue will be insufficient to create a substantiallytransmural lesion. If the energy source is too close, the energy maydamage the tissue or cause the energy source to overheat. Thus, there isa need for systems and methods that can account for the position of anenergy delivery device during ablation of tissue. Moreover, successfulablation is dependent on proper positioning of the ablation device.Because the target tissue can move during ablation (e.g. when ablatingtissue in a beating heart), it can also be difficult to create a lesionhaving the desired depth in the tissue. Additionally the motion of thetissue can result in damage of the tissue, insufficient ablation of thetissue, or damage to the device. Thus there is also a need for improvedsystems and methods that can accommodate tissue motion during ablationand that can provide information about the depth of the lesion. It wouldalso be desirable to provide an ablation system that is easy to use,easy to manufacture and that is lower in cost than current commercialsystems. At least some of these objectives will be met by the disclosureprovided herein.

2. Description of the Background Art

Patents related to the treatment of atrial fibrillation include, but arenot limited to the following: U.S. Pat. Nos. 6,997,925; 6,996,908;6,966,908; 6,964,660; 6,955,173; 6,954,977; 6,953,460; 6,949,097;6,929,639; 6,872,205; 6,814,733; 6,780,183; 6,666,858; 6,652,515;6,635,054; 6,605,084; 6,547,788; 6,514,249; 6,502,576; 6,416,511;6,383,151; 6,305,378; 6,254,599; 6,245,064; 6,164,283; 6,161,543;6,117,101; 6,064,902; 6,052,576; 6,024,740; 6,012,457; 5,405,346;5,314,466; 5,295,484; 5,246,438; and 4,641,649.

Patent Publications related to the treatment of atrial fibrillationinclude, but are not limited to International PCT Publication No. WO99/02096; and U.S. Patent Publication No. 2005/0267453. Scientificpublications related to the treatment of atrial fibrillation include,but are not limited to: Haissaguerre, M. et al., Spontaneous Initiationof Atrial Fibrillation by Ectopic Beats Originating in the PulmonaryVeins, New England J Med., Vol. 339:659-666; J. L. Cox et al., TheDevelopment of the Maze Procedure for the Treatment of AtrialFibrillation, Seminars in Thoracic & Cardiovascular Surgery, 2000; 12:2-14; J. L. Cox et al., Electrophysiologic Basis, Surgical Development,and Clinical Results of the Maze Procedure for Atrial Flutter and AtrialFibrillation, Advances in Cardiac Surgery, 1995; 6: 1-67; J. L. Cox etal., Modification of the Maze Procedure for Atrial Flutter and AtrialFibrillation. II, Surgical Technique of the Maze III Procedure, Journalof Thoracic & Cardiovascular Surgery, 1995; 110:485-95; J. L. Cox, N.Ad, T. Palazzo, et al. Current Status of the Maze Procedure for theTreatment of Atrial Fibrillation, Seminars in Thoracic & CardiovascularSurgery, 2000; 12: 15-19; M. Levinson, Endocardial Microwave Ablation: ANew Surgical Approach for Atrial Fibrillation; The Heart Surgery Forum,2006; Maessen et al., Beating Heart Surgical Treatment of AtrialFibrillation with Microwave Ablation, Ann Thorac Surg 74: 1160-8, 2002;A. M. Gillinov, E. H. Blackstone and P. M. McCarthy, AtrialFibrillation: Current Surgical Options and their Assessment, Annals ofThoracic Surgery 2002; 74:2210-7; Sueda T., Nagata H., Orihashi K., etal., Efficacy of a Simple Left Atrial Procedure for Chronic AtrialFibrillation in Mitral Valve Operations, Ann Thorac Surg 1997;63:1070-1075; Sueda T., Nagata H., Shikata H., et al.; Simple LeftAtrial Procedure for Chronic Atrial Fibrillation Associated with MitralValve Disease, Ann Thorac Surg 1996; 62:1796-1800; Nathan H., EliakimM., The Junction Between the Left Atrium and the Pulmonary Veins, AnAnatomic Study of Human Hearts, Circulation 1966; 34:412-422; Cox J. L.,Schuessler R. B., Boineau J. P., The Development of the Maze Procedurefor the Treatment of Atrial Fibrillation, Semin Thorac Cardiovasc Surg2000; 12:2-14; and Gentry et al., Integrated Catheter for 3-DIntracardiac Echocardiography and Ultrasound Ablation, IEEE Transactionson Ultrasonics, Ferroelectrics, and Frequency Control, Vol. 51, No. 7,pp 799-807.

BRIEF SUMMARY OF THE INVENTION

The present application generally relates to systems and methods forablating tissue. More specifically, the present application relates tothe treatment of fibrillation or other arrhythmias of the heart by usingultrasound energy, and even more specifically, the present applicationrelates to ablation systems and methods used to treat atrialfibrillation that provide information related to position of theablation device relative to the tissue, as well as providing informationabout depth of the lesion and motion of the energy source relative tothe tissue.

In a first aspect of the present invention, a method of ablating tissuein a patient as a treatment for fibrillation comprises providing anablation device having an energy source and a sensor. The energy sourceprovides a beam of energy directable to target tissue, and the sensorsenses energy reflected back from the target tissue. The sensor collectsa noise profile of the ablation device. The ablation device ispositioned adjacent the target tissue, and an amplitude mode data setthat characterizes the target tissue is collected with the sensor. Thenoise profile is removed from the amplitude mode data set to determine agap distance between the energy source and the target tissue. The targettissue is ablated with the beam of energy and operating parameters ofthe ablating step are adjusted in response to the gap distance.

In any aspect of the present invention, the energy source may comprisean ultrasound transducer. Positioning of the ablation device maycomprise advancing the ablation device into a left atrium of the heart.The ablating may create a contiguous lesion in the target tissue, andthe lesion may block aberrant electrical pathways in the tissue so as toreduce or eliminate the fibrillation. The fibrillation may compriseatrial fibrillation.

The noise profile may comprise an average amplitude value from a set ofsamples obtained in an echo-less environment. The noise profile may alsocomprise one of a signal pattern, a frequency band, and a set ofsignals. The noise profile may comprise electrical noise, backscatternoise of a fluid, or sensor noise. Collecting the noise profile maycomprise positioning at least a portion of the ablation device into afluid and collecting samples, or collection may be performed duringcalibration of the ablation device. Collecting of the noise profile mayfurther comprise storing the noise profile data.

The amplitude mode data set may be collected continuously orperiodically during the ablating, or during a during a diagnostic sweepof the target tissue prior to the ablating. Removing the noise may berepeated continuously or periodically during the ablating.

The method may further comprise using a plurality of gap distances toapproximate an incident angle of the beam of energy relative to asurface of the target tissue. Operating parameters of the ablation stepmay be adjusted in response to the approximated incident angle.

The adjusting may comprise adjusting the power in the beam of energy, orrepositioning the energy source relative to the target tissue.

In another aspect of the present invention, a method of ablating tissuein a patient as a treatment for fibrillation comprises providing anablation device having an energy source and a sensor. The energy sourceprovides a beam of energy directable to target tissue, and the sensorsenses energy reflected back from the target tissue. The ablation deviceis positioned adjacent the target tissue, and data is collected over asurface of the target tissue with the sensor. A map of the tissue isgenerated and the target tissue is ablated with the beam of energy. Thetissue map is used to facilitate execution of the ablation step.

The collecting step may comprise moving the sensor across the targettissue in any pattern, including a zig-zag pattern or a continuous path.Collecting the data may comprise collecting gap distance between theenergy source and a surface of the target tissue.

The map may be generated as a part of a diagnostic sweep of the targettissue prior to the ablating step, and the map may indentify anatomicalstructure in the target tissue such as the pulmonary veins. The map mayindicate gap distance between the energy source and a surface of thetarget tissue, or the map may indicate surface contours and angles ofthe target tissue. The map may comprise a two dimensional or a threedimensional representation of the target tissue.

Use of the tissue map may facilitate generation of an ablation path oradjustment of power in the beam of energy. The map may also facilitateplanning of ablation path distances. The map may comprise angles of theenergy beam relative to the target tissue, and use of the map mayfacilitate adjustment of power in the beam of energy based on theangles.

In still another aspect of the present invention, a method of ablatingtissue in a patient as a treatment for fibrillation comprises providingan ablation device having an energy source and a sensor. The energysource provides a beam of energy directable to target tissue, and thesensor senses energy reflected back from the target tissue. The ablationdevice is positioned adjacent the target tissue and a portion of thetarget tissue is repeatedly scanned. Information about the motion of thetarget tissue relative to the energy source is calculated. The targettissue is ablated with the beam of energy and the motion is accountedfor during the ablation.

The scanning may comprise collecting information about gap distancebetween the energy source and a surface of the target tissue. The targettissue may comprise tissue in the heart, and the scanning may occurwhile the heart is beating. Scanning may comprise scanning the targettissue over a short time duration of 5 milliseconds or less. Scanningmay comprise repeatedly scanning a single spot.

The calculating may comprise calculating variance, velocity, oracceleration of the target tissue. Accounting for the motion maycomprise adjusting the energy beam position or power based on thecalculated information. The accounting step may also comprisemaintaining gap distance between the energy source and the target tissuewithin a predetermined range, based on the calculated information. Thepredetermined range may be from 2 and 20 mm.

The method may further comprise identifying tissue type based on themotion. Sensitive tissue that is unsuitable for ablation may beidentified based on the motion. Anatomical structures such as thepulmonary veins may be identified based on the motion. The method mayfurther comprise determining thickness of the target tissue based on themotion.

In yet another aspect of the present invention, a method for of ablatingtissue in a patient as a treatment for fibrillation comprises providingan ablation device having an energy source and a sensor. The energysource provides a beam of energy directable to target tissue, and thesensor senses energy reflected back from the target tissue. The ablationdevice is positioned adjacent the target tissue and a standard lesionratio from the target tissue is provided. An initial backscatter signalfrom unablated target tissue is sensed with the sensor and the targettissue is ablated with the beam of energy. A post ablation backscattersignal from the target tissue after ablation is sensed. The currentlesion ratio is then compared to the standard lesion ratio, and theablation is discontinued when the current lesion ratio is greater thanor equal to the standard lesion ratio.

The standard lesion ratio may comprise a numerical value associated witha substantially transmural lesion. The standard lesion ratio maycomprise a normalized backscatter signal value of a transmural lesion.The standard lesion ratio may also comprise a normalized signal pattern,a frequency, or other unique property of a substantially transmurallesion. Providing the standard lesion ratio may further comprise sensinga tissue backscatter signal in a region of the tissue with highechodensity, sensing a tissue backscatter signal in a region of thetissue with low echodensity, and determining a ratio of the high and thelow echodensity backscatter signals. The region of high echodensity maycomprise a substantially transmural lesion, and the region of lowechodensity may comprise unablated tissue. The standard lesion ratio maycomprise a laboratory determined value.

The sensing of the initial backscatter signal may be obtained during adiagnostic sweep of the target tissue prior to ablation thereof, or itmay be obtained prior to the ablating, or during the ablating. Theablating may comprise incrementally increasing lesion depth in theablated tissue. The increments may decrease in value as lesion depthincreases or the increments may be of constant value. The ablating maycomprise continuously increasing lesion depth in the ablated tissue.

The sensing of the post ablation backscatter signal may occur after eachincremental ablation step is performed, or it may occur periodicallyduring the ablation. It may also occur continuously during the ablation.The comparing step may determine if the ablation has created asubstantially transmural lesion in the target tissue.

These and other embodiments are described in further detail in thefollowing description related to the appended drawing figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an exemplary embodiment of an energy delivery device.

FIG. 2 illustrates exemplary use of the energy delivery device in FIG. 1to ablate cardiac tissue.

FIG. 3 illustrates an exemplary embodiment of the energy source andbacking.

FIGS. 4A-4B illustrate alternative embodiments of an energy source.

FIGS. 5-6 illustrate still other embodiments of an energy source.

FIG. 7 illustrates the energy beam and zone of ablation in tissue.

FIGS. 8A-8D illustrate the energy beam and zone of ablation in tissue.

FIGS. 9-10 illustrate the energy beam and zone of ablation in tissue.

FIG. 11 illustrates a flowchart of an exemplary method for collectinggap data.

FIG. 12 illustrates a flowchart of an exemplary method for pre-mappingtissue.

FIG. 13 illustrates a flowchart of an exemplary method for accommodatingtissue motion.

FIG. 14 illustrates a flowchart of an exemplary method for monitoringlesion depth during ablation.

DETAILED DESCRIPTION OF THE INVENTION

The following description of the preferred embodiments of the inventionis not intended to limit the invention to these preferred embodiments,but rather to enable any person skilled in the art to make and use thisinvention.

As shown in FIG. 1, the energy delivery system 10 of the preferredembodiments includes an energy source 12, that functions to provide asource of ablation energy, and an electrical attachment 14, coupled tothe energy source 12, that functions to energize the energy source 12such that it emits an energy beam 20. The energy delivery system 10 ofthe preferred embodiments also includes a sensor 40 or the energy source12 may also serve as the sensor to detect the gap (distance of thetissue surface from the energy source 12), the thickness of the tissuetargeted for ablation, the characteristics of the ablated tissue, andany other suitable parameter or characteristic of the tissue and/or theenvironment around the energy delivery system 10. The energy deliverysystem 10 of the preferred embodiments also includes a processor 33operatively coupled to the sensor and to the electrical attachment 14,that controls the electrical attachment 14 and/or the electrical signaldelivered to the energy source 12 based on the information from thesensor 40. The energy delivery system 10 is preferably designed fordelivering energy to tissue, more specifically, for delivering ablationenergy to tissue, such as heart tissue, to create a conductionblock—isolation and/or block of conduction pathways of abnormalelectrical activity, which typically originate from the pulmonary veinsin the left atrium for treatment of atrial fibrillation in a patient.The system 10, however, may be alternatively used with any suitabletissue in any suitable environment and for any suitable reason.

The Energy Source. As shown in FIG. 1, the energy source 12 of thepreferred embodiments functions to provide a source of ablation energyand emit an energy beam 20. The energy source 12 is preferably moved andpositioned within a patient, preferably within the left atrium of theheart of the patient, such that the energy source 12 is positioned at anappropriate angle with respect to the target tissue. The angle ispreferably any suitable angle such that the emitted energy beam 20propagates into the target tissue, and preferably generates a transmurallesion (i.e. a lesion through the thickness of the tissue thatpreferably creates a conduction block, as described below). Anglesbetween 40 and 140 degrees are preferable because in this range themajority of the energy beam will preferably propagate into the tissueand the lesion depth needed to achieve transmurality is preferablyminimally increased from the ideal orthogonal angle.

As shown in FIG. 1, the energy source 12 is preferably coupled to ahousing 16. The energy source 12 and the housing 16 are preferablypositionable within the patient. For example, the housing 16, and theenergy source 12 within it, are preferably moved to within the leftatrium of the heart (or in any other suitable location) and, oncepositioned there, are preferably moved to direct the energy source 12and the emitted energy beam 20 towards the target tissue at anappropriate angle. Furthermore, the housing 16, and the energy source 12within it, are preferably moved along an ablation path such that theenergy source 12 provides a partial or complete zone of ablation alongthe ablation path. The zone of ablation along the ablation pathpreferably has any suitable geometry to provide therapy, such asproviding a conduction block for treatment of atrial fibrillation in apatient. The zone of ablation along the ablation path may alternativelyprovide any other suitable therapy for a patient. A linear ablation pathis preferably created by moving the housing 16, and the energy source 12within it, along an X, Y, and/or Z-axis. As shown in FIG. 2, the motionof the distal portion of the elongate member 18 in and out of the guidesheath portion GS of the elongate member 18 is represented by thez-axis. A generally circular or elliptical ablation path is preferablycreated by rotating the energy source 12 about an axis (for example, asdefined by the wires W in FIG. 2). The elongate member 18, along withthe housing 16 and the energy source 12, is preferably rotated, as shownin FIG. 2. Alternatively, in other configurations, the energy source 12is rotated within the housing 16. For example, as shown in FIG. 2, thehousing 16 points towards the wall tissue 2174 of an atrium. The energysource 12 in the housing 16 emits an energy beam to establish anablation window 2172. As the housing 16 (and an elongate member 18,described below) are rotated (as shown by arrow 2124 in FIG. 2), theablation window 2172 sweeps a generally circular ablation path 2176creating a section of a conical shell. Further, in this example, it maybe desirable to move the elongate member forwards or backwards along theZ-axis to adjust for possible variations in the anatomy. Although theablation path is preferably linear or circular, any suitable ablationpath may be created by any suitable combination of movement in the X, Y,and Z axes and rotational movement.

As shown in FIG. 1, the energy delivery system 10 of the preferredembodiments may also include an elongate member 18, coupled to theenergy source 12. The elongate member 18 is preferably a catheter madeof a flexible multi-lumen tube, but may alternatively be a cannula, tubeor any other suitable elongate structure having one or more lumens. Theelongate member 18 of the preferred embodiments functions to accommodatepull wires, fluids, gases, energy delivery structures, electrical wires,therapy catheters, navigation catheters, pacing catheters, connectionsand/or any other suitable device or element. As shown in FIG. 1, theelongate member 18 preferably includes a housing 16 positioned at adistal portion of the elongate member 18. The elongate member 18 furtherfunctions to move and position the energy source 12 and/or the housing16 within a patient, such that the emitted energy beam 20 propagatesinto the target tissue at an appropriate angle and the energy source 12and/or the housing 16 is moved along an ablation path such that theenergy source 12 provides a partial or complete zone of ablation alongthe ablation path.

The energy source 12 is preferably an ultrasound transducer that emitsan ultrasound beam, but may alternatively be any suitable energy sourcethat functions to provide a source of ablation energy. Suitable sourcesof ablation energy include but are not limited to, radio frequency (RF)energy, microwaves, photonic energy, and thermal energy. The therapycould alternatively be achieved using cooled sources (e.g., cryogenicfluid). The energy delivery system 10 preferably includes a singleenergy source 12, but may alternatively include any suitable number ofenergy sources 12. The ultrasound transducer is preferably made of apiezoelectric material such as PZT (lead zirconate titanate) or PVDF(polyvinylidine difluoride), or any other suitable ultrasound emittingmaterial. For simplicity, the front face of the transducer is preferablyflat, but may alternatively have more complex geometry such as eitherconcave or convex to achieve an effect of a lens or to assist inapodization—selectively decreasing the vibration of a portion orportions of the surface of the transducer—and management of thepropagation of the energy beam 20. The transducer preferably has acircular geometry, but may alternatively be elliptical, polygonal, orany other suitable shape. The transducer may further include coatinglayers which are preferably thin layer(s) of a suitable material. Somesuitable transducer coating materials may include graphite, metal-filledgraphite, gold, stainless steel, magnesium, nickel-cadmium, silver, anda metal alloy. For example, as shown in FIG. 1, the front face of theenergy source 12 is preferably coupled to one or more matching layers34. The matching layer(s) preferably functions to increase theefficiency of coupling of the energy beam 20 into the surrounding fluid28. The matching layer 34 is preferably made from a plastic such asparylene, preferably placed on the transducer face by a vapor depositiontechnique, but may alternatively be any suitable material, such asgraphite, metal-filled graphite, metals, or ceramic, added to thetransducer in any suitable manner.

The energy source 12 is preferably one of several variations. In a firstvariation, as shown in FIG. 3, the energy source 12 is a disc with aflat front surface coupled to a backing 22 with an adhesive ring 24. Thebacking 22 forms a pocket 26 to help reflect energy in a desireddirection, often distally away from the housing 16 into the treatmenttissue. A plurality of axial channel or grooves 36 along the backingallow fluid to flow therepast in order to help cool the transducer 12and prevent direct tissue contact. In a second variation, as shown inFIGS. 4A and 4B, the energy source 12′ includes an inactive portion 42.In this variation, the inactive portion 42 does not emit an energy beamwhen the energy source 12 is energized, or may alternatively emit anenergy beam with a very low (substantially zero) energy. The inactiveportion 42 preferably functions to aid in the temperature regulation ofthe energy source, i.e. preventing the energy source from becoming toohot. In a full disk transducer, as shown in FIG. 3, the center portionof the transducer generally becomes the hottest portion of thetransducer while energized. By removing the center portion or a portionof the center portion of the transducer, the energy emitted from thetransducer is preferably distributed differently across the transducer,and the heat of the transducer is preferably more easily dissipated.

The inactive portion 42 is preferably a hole or gap defined by theenergy source 12′. In this variation, a coolant source may be coupledto, or in the case of a coolant fluid, it may flow through the hole orgap defined by the energy source 12′ to further cool and regulate thetemperature of the energy source 12′. The inactive portion 42 mayalternatively be made of a material with different material propertiesfrom that of the energy source 12′. For example, the material ispreferably a metal, such as copper, which functions to draw or conductheat away from the energy source 12. Alternatively, the inactive portionis made from the same material as the energy source 12, but with theelectrode plating removed or disconnected from the electricalattachments 14 and or the generator. The inactive portion 42 ispreferably disposed along the full thickness of the energy source 12′,but may alternatively be a layer of material on or within the energysource 12′ that has a thickness less than the full thickness of theenergy source 12′. As shown in FIG. 4A, the energy source 12′ ispreferably a doughnut-shaped transducer. As shown, the transducerpreferably defines a hole (or inactive portion 42) in the center portionof the transducer. The energy source 12′ of this variation preferablyhas a circular geometry, but may alternatively be elliptical, polygonalas shown in FIG. 4B), or any other suitable shape. The energy source 12′preferably includes a singular, circular inactive portion 42, but mayalternatively include any suitable number of inactive portions 42 of anysuitable geometry, as shown in FIG. 4B. The total energy emitted fromthe energy source 12 is related to the surface area of the energy source12 that is active (i.e. emits energy beam 20). Therefore, the size andlocation of inactive portion(s) 42 preferably sufficiently reduce theheat build-up in the energy source 12, while allowing the energy source12 to provide as much output energy as possible or as desired.

In a third variation, as shown in FIG. 5, the energy source 12″preferably includes a plurality of annular transducers 44. The pluralityof annular transducers is preferably a plurality concentric rings, butmay alternatively have any suitable configuration with any suitablegeometry, such as elliptical or polygonal. The energy source 12″ mayfurther include an inactive portion 42, such as the center portion ofthe energy source 12″ as shown in FIG. 5. The plurality of annulartransducers 44 preferably includes at least a first annular transducerand a second annular transducer. The first annular transducer preferablyhas material properties that differ from those of the second annulartransducer, such that the first annular transducer emits a first energybeam that is different from the second energy beam emitted from thesecond annular ring. Furthermore, the first annular transducer may beenergized with a different frequency, voltage, duty cycle, power, and/orfor a different length of time from the second annular transducer.Alternatively the first annular ring may be operated in a different modefrom the second annular ring. For example, the first annular ring may berun in a therapy mode, such as ablate mode which delivers a pulse ofultrasound sufficient for heating of the tissue, while the secondannular ring may be run in a diagnostic mode, such as A-mode, whichdelivers a pulse of ultrasound of short duration, which is generally notsufficient for heating of the tissue but functions to detectcharacteristics of the target tissue and/or environment in and aroundthe energy delivery system. The first annular transducer may furtherinclude a separate electrical attachment 14 from that of the secondannular transducer.

In a fourth variation, as shown in FIG. 6, the energy source 12″′preferably includes a grid of transducer portions 46. The grid oftransducer portions 46 preferably has any suitable geometry such ascircular, rectangular (as shown in FIG. 6), elliptical, polygonal, orany other suitable geometry. The energy source 12″′ in this variationmay further include a transducer portion that is inactive, such as aninactive portion as described in the second variation of the energysource 12′. The grid of transducer portions 46 preferably includes atleast a first transducer portion and a second transducer portion. In afirst version, the first transducer portion and the second transducerportion are preferably portions of a single transducer with a single setof material properties. The first transducer portion is preferablyenergized with a different frequency, voltage, duty cycle, power, and/orfor a different length of time from the second transducer portion.Furthermore the first transducer portion may be operated in a differentmode from the second transducer portion. For example, the firsttransducer portion may operate in a therapy mode, such as ablate mode,while the second transducer portion may operate in a diagnostic mode,such as A-mode. In this version, the first transducer portion mayfurther include a separate electrical attachment 14 from that of thesecond transducer portion. For example, the first transducer portion maybe located towards the center of the energy source 12″′ and the secondtransducer portion may be located towards the outer portion of theenergy source 12″′ and the second transducer portion may be energizedwhile the first transducer portion remains inactive. In a secondversion, the first transducer portion preferably has material propertiesthat differ from those of the second transducer portion, such that thefirst transducer portion emits a first energy beam that is differentfrom the second energy beam emitted from the second portion. In thisversion, the first transducer portion may also be energized with adifferent frequency, voltage, duty cycle, power, and/or for a differentlength of time from the second transducer portion.

The Electrical Attachment. As shown in FIG. 1, the electrical attachment14 of the preferred embodiments functions to energize the energy source12 such that it emits an energy beam 20. In use, as the energy source 12is energized, it emits an energy beam 20 towards targeted tissue. As theenergy is transferred from the energy beam 20 into the tissue, thetargeted tissue portion is preferably heated sufficiently to achieveablation. As shown in FIG. 1, the electrical attachment 14 is preferablycoupled to the energy source 12. The energy delivery system 10preferably includes two electrical attachments 14 and 14′, but mayalternatively include any suitable number of electrical attachments toenergize the energy source 12. The energy source 12 preferably has afirst electrical attachment 14 coupled the front surface of the energysource 12 which is coupled to a suitably insulated wire 38. Theelectrical attachment 14 is preferably accomplished by standard bondingtechniques such as soldering, wire bonding, conductive epoxy, orswaging. The electrical attachment 14 is preferably placed closer to theedge of the energy source 12 so as not to disturb the energy beam 20emitted by the energy source 12 upon being electrically energized. Theenergy source 12 preferably has a second electrical attachment 14′coupled to the back surface of the energy source 12 which is coupled toa suitably insulated wire 38′. Wires 38 and 38′ together form a pair38″, which are preferably a twisted shielded pair, a miniature coaxialcable, a metal tube braid, or are coupled in any other suitable method.The electrical attachment(s) 14 may alternatively be coupled to theenergy source 12 in any other suitable fashion in any other suitableconfiguration.

The energy delivery system 10 of the preferred embodiments also includesan electrical generator (not shown) that functions to provide power tothe energy source 12 via the electrical attachment(s) 14. The energysource 12 is preferably coupled to the electrical generator by means ofthe suitably insulated wires 38 and 38′ connected to the electricalattachments 14 and 14′ coupled to the two faces of the energy source 12.When energized by the generator the energy source 12 emits energy. Thegenerator provides an appropriate signal to the energy source 12 tocreate the desired energy beam 20. The frequency is preferably in therange of 5 to 25 MHz, more preferably in the range of 8 to 20 MHz, andeven more preferably in the range of 2 to 15 MHz. The energy of theenergy beam 20 is determined by the excitation voltage applied to theenergy source 12, the duty cycle, and the total time the voltage isapplied. The voltage is preferably in the range of 5 to 200 voltspeak-to-peak. In addition, a variable duty cycle is preferably used tocontrol the average power delivered to the energy source 12. The dutycycle preferably ranges from 0% to 100%, with a repetition frequencythat is preferably faster than the time constant of thermal conductionin the tissue. One such appropriate repetition frequency isapproximately 40 kHz.

Energy Beam and Tissue Interaction. When energized with an electricalsignal or pulse train by the electrical attachment 14 and/or 14′, theenergy source 12 emits an energy beam 20 (such as a sound pressurewave). The properties of the energy beam 20 are determined by thecharacteristics of the energy source 12, the matching layer 34, thebacking 22 (described below), the electrical signal from electricalattachment 14. These elements determine the frequency, bandwidth, andamplitude of the energy beam 20 (such as a sound wave) propagated intothe tissue. As shown in FIG. 7, the energy source 12 emits energy beam20 such that it interacts with tissue 276 and forms a lesion (zone ofablation 278). The energy beam 20 is preferably an ultrasound beam. Thetissue 276 is preferably presented to the energy beam 20 within thecollimated length L. The front surface 280 of the tissue 276 is at adistance d (282) away from the distal face of the housing 16. As theenergy beam 20 travels through the tissue 276, its energy is absorbedand scattered by the tissue 276 and most of the ablation energy isconverted to thermal energy. This thermal energy heats the tissue totemperatures higher than the surrounding tissue resulting in a heatedzone 278. In the zone 278 where the tissue is heated, the tissue cellsare preferably rendered dead due to heat. The temperatures of the tissueare preferably above the temperature where cell death occurs in theheated zone 278 and therefore, the tissue is said to be ablated. Hence,the zone 278 is preferably referenced as the ablation zone or lesion.

The Physical Characteristics of the Lesion. The shape of the lesion orablation zone 278 formed by the energy beam 20 depends on thecharacteristics of suitable combination factors such as the energy beam20, the energy source 12 (including the material, the geometry, theportions of the energy source 12 that are energized and/or notenergized, etc.), the matching layer 34, the backing 22 (describedbelow), the electrical signal from electrical attachment 14 (includingthe frequency, the voltage, the duty cycle, the length and shape of thesignal, etc.), and the characteristics of target tissue that the beam 20propagates into and the length of contact or dwell time. Thecharacteristics of the target tissue include the thermal transferproperties and the ultrasound absorption, attenuation, and backscatterproperties of the target tissue and surrounding tissue.

The shape of the lesion or ablation zone 278 formed by the energy beam20 is preferably one of several variations due to the energy source 12(including the material, the geometry, the portions of the energy source12 that are energized and/or not energized, etc.). In a first variationof the ablation zone 278, as shown in FIG. 7, the energy source 12 is afull disk transducer and the ablation zone 278 is a tear-shaped lesion.The diameter D1 of the zone 278 is smaller than the diameter D of thebeam 20 at the tissue surface 280 and further, the outer layer(s) oftissue 276 preferably remain substantially undamaged. This is due to thethermal cooling provided by the surrounding fluid (cooling fluid and/orblood), which is flowing past the tissue surface 280. More or less ofthe outer layers of tissue 276 may be spared or may remain substantiallyundamaged due to the amount that the tissue surface 280 is cooled and/orthe characteristics of the energy delivery system 10 (including theenergy source 12 and the energy beam 20). The energy deposited in theablation zone 278 preferably interacts with the non-surface layer(s) oftissue such that the endocardial surface remains pristine (and/or notcharred). As the energy beam 20 travels deeper into the tissue, thethermal cooling is provided by the surrounding tissue, which is not asefficient as that on the surface. The result is that the ablation zone278 has a larger diameter D2 than D1 as determined by the heat transfercharacteristics of the surrounding tissue as well as the continued inputof the energy from the beam 20. As the beam 20 is presented to thetissue for an extended period of time, the ablation zone 278 extendsinto the tissue, but not indefinitely. There is a natural limit of thedepth 288 of the ablation zone 278 as determined by the factors such asthe attenuation and absorption of the ultrasound energy as the energybeam 20 propagates into the tissue, heat transfer provided by thehealthy surrounding tissue, and the divergence of the beam beyond thecollimated length L. During this ultrasound-tissue interaction, theultrasound energy is being absorbed by the tissue, and therefore lessand less of it is available to travel further into the tissue. Thus acorrespondingly smaller diameter heated zone is developed in the tissue,and the overall result is the formation of the heated ablation zone 278,which is in the shape of an elongated tear limited to a depth 288 intothe tissue.

In a second variation, as shown in FIG. 9, the ablation zone 278′ has ashorter depth 288′. In this variation, the lesion preferably has a moreblunt shape than ablation zone 278 (FIG. 7). One possible lesiongeometry of this second variation may be a tooth shaped geometry, asshown in FIG. 9, but may alternatively have any suitable shape such as ablunt tear shape, a circular shape, or an elliptical shape. As shown inFIG. 9, zone 278′ (similarly to zone 278 in FIG. 7) has a diameter D1 ofthe zone 278′ smaller than the diameter D of the beam 20 at the tissuesurface 280 due to the thermal cooling provided by the surrounding fluidflowing past the tissue surface 280. In this variation, the energysource 12′ preferably has an inactive portion 42 located at the centerof the energy source 12′, such that energy source is a doughnut-shapedtransducer which emits an energy beam 20 that is generally morediffused, with a broader, flatter profile, than the energy beam 20 ofthe first variation (FIG. 7). The energy beam 20 emitted from thedoughnut-shaped transducer, as shown in FIG. 9, preferably has a reducedpeak intensity along the midline of the energy beam (as shown in crosssection by the dotted lines in FIG. 9). With this ultrasound-tissueinteraction, the reduced peak intensity along the midline of the energybeam is being absorbed by the tissue, and less and less of the energy isavailable to travel further into the tissue, forming a blunter lesionthan in the first variation.

The size and characteristics of the ablation zone also depend on thefrequency and voltage applied to the energy source 12 to create thedesired energy beam 20. For example, as the frequency increases, thedepth of penetration of ultrasound energy into the tissue is reducedresulting in an ablation zone 278 (ref. FIG. 7) of shallower depth 288.The frequency is preferably in the range of 5 to 25 MHz, more preferablyin the range from 8 to 20 MHz, and even more preferably in the rangefrom 10 to 18 MHz. The energy of the energy beam 20 is determined by theexcitation voltage applied to the energy source 12 for a transducerfabricated from PZT material, for example. The voltage is preferably inthe range of 5 to 200 volts peak-to-peak. In addition, a variable dutycycle is preferably used to control the average power delivered to theenergy source 12. The duty cycle preferably ranges from 0% to 100%, witha repetition frequency of approximately 40 kHz, which is preferablyfaster than the time constant of thermal conduction in the tissue. Thisresults in an ablation zone 278, which is created within 1 to 5 seconds,and is of depth 288 of approximately 5 millimeters (mm), and of amaximum diameter of approximately 2.5 mm in correspondence to thediameter of the energy source 12, for an average power level preferably0.5 to 25 watts, more preferably 2 to 10 watts, and even more preferably2 to 7 watts.

The size and characteristics of the ablation zone 278 also depend on thetime the targeted tissue is contacted by the energy beam 20, as shown inFIGS. 8A-8D, which exemplify the formation of the lesion at times t₁,t₂, t₃ and t₄, respectively. The ablation zone 278 in the tissue isformed by the conversion of the ultrasound energy to thermal energy inthe tissue. As the energy beam 20 initially impinges on the frontsurface 280 of the tissue 276 at time t.sub.1, heat is created whichbegins to form the lesion 278 (FIG. 8A). As time passes on to t₂, and t₃(FIGS. 8B and 8C), the ablation zone 278 continues to grow in diameterand depth. This time sequence from t₁ to t₃ preferably takes as littleas 1 to 5 seconds, depending on the ultrasound energy density. As theincidence of the ultrasound beam is continued beyond time t₃, theablation lesion 278 grows slightly in diameter and length, and thenstops growing due to the steady state achieved in the energy transferfrom its ultrasound form to the thermal form balanced by the dissipationof the thermal energy into the surrounding tissue. The example shown inFIG. 8D shows the lesion after an exposure t₄ of approximately 30seconds to the energy beam 20. Thus the lesion reaches a natural limitin size and does not grow indefinitely.

The ultrasound energy density preferably determines the speed at whichthe ablation occurs. The acoustic power delivered by the energy source12 divided by the cross sectional area of the beam 20 determines theenergy density per unit time. Effective acoustic power preferably rangesfrom 0.5 to 25 watts, more preferably from 2 to 10 watts, and even morepreferably from 2 to 7 watts, and the corresponding power densitiespreferably range from 50 watts/cm² to 2500 watts/cm². These powerdensities are developed in the ablation zone. As the beam divergesbeyond the ablation zone, the power density falls such that ablationwill not occur, regardless of the time exposure.

Although the shape of the ablation zone 278 is preferably one of severalvariations, the shape of the ablation zone 278 may be any suitable shapeand may be altered in any suitable fashion due to any suitablecombination of the energy beam 20, the energy source 12 (including thematerial, the geometry, etc.), the matching layer 34, the backing 22(described below), the electrical signal from electrical attachment 14(including the frequency, the voltage, the duty cycle, the length of thepulse, etc.), and the target tissue the beam 20 propagates into and thelength of contact or dwell time.

The Sensor. The energy delivery system 10 of the preferred embodimentsalso includes a sensor separate from the energy source and/or the energysource 12 may further function as a sensor to detect the gap (thedistance of the tissue surface from the energy source 12), the thicknessof the tissue targeted for ablation, the characteristics of the ablatedtissue, the incident beam angle, and any other suitable parameter orcharacteristic of the tissue and/or the environment around the energydelivery system 10, such as the temperature. By detecting theinformation, the sensor (coupled to the processor, as described below)preferably functions to guide the therapy provided by the ablation ofthe tissue.

The sensor is preferably one of several variations. In a firstvariation, the sensor is preferably an ultrasound transducer thatfunctions to detect information with respect to the gap, the thicknessof the tissue targeted for ablation, the characteristics of the ablatedtissue, and any other suitable parameter or characteristic. The sensorpreferably has a substantially identical geometry as the energy source12 to insure that the area diagnosed by the sensor is substantiallyidentical to the area to be treated by the energy source 12. Morepreferably, the sensor is the same transducer as the transducer of theenergy source, wherein the energy source 12 further functions to detectinformation by operating in a different mode (such as A-mode, definedbelow).

The sensor of the first variation preferably utilizes a burst ofultrasound of short duration, which is generally not sufficient forheating of the tissue. This is a simple ultrasound imaging technique,referred to in the art as A Mode, or Amplitude Mode imaging. As shown inFIG. 10, sensor 40 preferably sends a burst 290 of ultrasound towardsthe tissue 276. A portion of the beam is reflected and/or backscatteredas 292 from the front surface 280 of the tissue 276. This returningsound wave 292 is detected by the sensor 40 a short time later andconverted to an electrical signal, which is sent to the electricalreceiver (not shown). The returning sound wave 292 is delayed by theamount of time it takes for the sound to travel from the sensor 40 tothe front boundary 280 of the tissue 276 and back to the sensor 40. Thistravel time represents a delay in receiving the electrical signal fromthe sensor 40. Based on the speed of sound in the intervening media(fluid 286 and blood 284), information regarding the gap distance d(282) is detected. As the sound beam travels further into the tissue276, a portion 293 of it is scattered from the lesion 278 being formedand travels towards the sensor 40. Again, the sensor 40 converts thissound energy into electrical signals and a processor (described below)converts this information into characteristics of the lesion formationsuch as thickness, etc. As the sound beam travels still further into thetissue 276, a portion 294 of it is reflected from the back surface 298and travels towards the transducer. Again, the sensor 40 converts thissound energy into electrical signals and the processor converts thisinformation into the thickness t (300) of the tissue 276 at the point ofthe incidence of the ultrasound burst 290. As the catheter housing 16 istraversed in a manner 301 across the tissue 276, the sensor 40 detectsthe gap distance d (282), lesion characteristics, and the tissuethickness t (300). The sensor preferably detects these parameterscontinuously, but may alternatively detect them periodically or in anyother suitable fashion. This information is used to manage the deliveryof continuous ablation of the tissue 276 during therapy as discussedbelow.

In a second variation, the sensor is a temperature sensor that functionsto detect the temperature of the target tissue, the surroundingenvironment, the energy source 12, the coolant fluid as described below,and/or the temperature of any other suitable element or area. Thetemperature senor is preferably a thermocouple, but may alternatively beany suitable temperature sensor, such as a thermistor or an infraredtemperature sensor. This temperature information gathered by the sensoris preferably used to manage the delivery of continuous ablation of thetissue 276 during therapy and to manage the temperature of the targettissue and/or the energy delivery system 10 as discussed below.

The Processor. The energy delivery system 10 of the preferredembodiments also includes a processor 33 (illustrated in FIG. 1),coupled to the sensor 40 and to the electrical attachment 14, thatcontrols the electrical attachment 14 and/or the electrical signaldelivered to the electrical attachment 14 based on the information fromthe sensor 40. The processor 33 is preferably a conventional processor,but may alternatively be any suitable device to perform the desiredfunctions.

The processor 33 preferably receives information from the sensor such asinformation related to the gap distance, the thickness of the tissuetargeted for ablation, the characteristics of the ablated tissue, andany other suitable parameter or characteristic. Based on thisinformation, the processor preferably controls the energy beam 20emitted from the energy source 12 by modifying the electrical signalsent to the energy source 12 via the electrical attachment 14 such asthe frequency, the voltage, the duty cycle, the length of the pulse,and/or any other suitable parameter. The processor preferably alsocontrols the energy beam 20 by controlling portions of the energy source12 that are energized using various frequencies, voltages, duty cycles,etc. Different portions of the energy source 12 may be energized asdescribed above with respect to the plurality of annular transducers 44and the grid of transducer portions 46 of the energy source 12″ and 12″′respectively. Additionally, the processor may further be coupled to afluid flow controller. The processor preferably controls the fluid flowcontroller to increase or decrease fluid flow based on the sensordetecting characteristics of the ablated tissue, of the unablated ortarget tissue, the temperature of the tissue and/or energy source, and/or the characteristics of any other suitable condition.

By controlling the energy beam 20 (and/or the cooling of the targetedtissue or energy source 12), the shape of the ablation zone 278 iscontrolled. For example, the depth 288 of the ablation zone ispreferably controlled such that a transmural lesion (a lesion throughthe thickness of the tissue) is achieved. Additionally, the processorpreferably functions to minimize the possibility of creating a lesionbeyond the targeted tissue, for example, beyond the outer atrial wall.If the sensor detects the lesion and/or the ablation window 2172 (asshown in FIG. 2) extending beyond the outer wall of the atrium or thatthe depth of the lesion has reached or exceeded a preset depth, theprocessor preferably turns off the generator and/or ceases to sendelectrical signals to the electrical attachment(s) 14.

Additionally, the processor preferably functions to maintain a preferredgap distance between the energy source and the tissue to be treated. Thegap distance is preferably between 2-25 mm, more preferably between 2-20mm, and even more preferably between 2-15 mm. If the sensor detects thelesion and/or the ablation window 2172 (as shown in FIG. 2) extendingbeyond the outer wall of the atrium or if it does not reach the outerwall of the atrium, or that the depth of the lesion has either notreached or has exceeded a preset depth, the processor preferablyrepositions the energy delivery system. For example, as the housing 16(and an elongate member 18, described below) are rotated (as shown byarrow 2124 in FIG. 2), the ablation window 2172 preferably sweeps agenerally circular ablation path 2176 creating a section of a conicalshell. However, if the sensor determines that the ablation window 2172is not reaching the wall of the atrium, the processor preferably movesthe elongate member forwards or backwards along the Z-axis, or indicatesthat it should be moved, to adjust for the possible variations in theanatomy. In this example, the operator can reposition the elongatemember, or the processor is preferably coupled to a motor drive unit orother control unit that functions to position the elongate member 18.

Gap Data Collection Method. As shown in FIG. 11, a preferred method tocollect noise-reduced gap data in an energy delivery system includescollecting a noise profile of an energy delivery system S 100,collecting an A-mode data set S 110, and removing noise profilecomponent from the A-mode data set S 120. The noise-reduced gap datamethod preferably functions to measure the gap distance (the distancebetween the tissue surface and the energy source) with a reduced signalto noise ratio. The collection of noise-reduced gap data preferablyoccur continuously or periodically during the ablation of tissue, butthe noise-reduced gap data may alternatively be collected during adiagnostic sweep of the tissue prior to an ablation sweep or at anyother suitable time.

Step S 100, which recites collecting a noise profile of an energydelivery system, functions to collect a set of data that represents thenoise signal received during normal measurements. The noise profile ispreferably an average amplitude value from a set of samples obtained inan echo-less environment (not during normal operation conditions). Thenoise profile may also be a signal pattern, a frequency band, a set ofsignals, or any other parameter that may be recognized as the noisecomponent. During A-mode sampling, the echo component is the desiredsignal. Thus, by measuring the noise profile, the processor can isolatethe echo component (the portion of the signal that has been reflectedoff a tissue surface) from the data. The noise profile may be a signalcomposed of electrical noise, backscatter noise of a fluid,sensor/component noise, or any other type of noise or combination ofnoise signals that are not a part of the echo component.

The step of collecting a noise profile is preferably performed bypositioning the energy delivery system 10 in a dish of saline solutionand collecting samples. The energy delivery system 10 may alternativelybe positioned or held in any suitable solution or material to create orsimulate an echoless environment. More preferably, Step S100 isperformed during a calibration mode and it includes storing the data asa noise profile. The calibration mode is preferably activated by theuser or alternatively may be automatically run prior to operation oractivated in any suitable manner. Alternatively, the processor mayaverage a large number of A-mode lines while the energy delivery system10 is deployed. In this alternative, any return signals from a realtarget is randomly located and over time would preferably average out toa baseline noise profile. The A-mode lines used for the averaged noiseprofile may be from a set number of samples during the lifetime use ofthe energy delivery system, or may alternatively be from an initialcalibration sweep performed at the beginning of a procedure.

Step S110, which recites collecting an A-mode data set, functions tocollect the normal diagnostic data. Step S110 is preferably repeatedcontinuously or periodically during an ablation process. Step S110 ispreferably performed in a manner identical to that described above withrespect to the sensor.

Step S120, which recites removing the noise profile component from theA-mode data set, functions to isolate the gap distance signal from thenoise. Preferably, the average value of the noise profile is subtractedfrom the A-mode data set. The result is preferably a signal dependent onthe gap distance. Alternatively, if the noise profile component iscontained within a certain frequency bandwidth, the processor preferablyfilters (with, for example, a lowpass, highpass, bandpass, or bandstopfilter) the signal to isolate the gap distance signal. Any othersuitable signal processing method may be used to isolate the gapdistance signal.

Steps S110 and S120 are preferably repeated continuously or periodicallyduring the ablation of tissue. Repeating Steps S110 and S120 functionsto collect the gap data during the ablation process so the gap distancemay be known at the time of the ablation process. The gap data mayalternatively be collected during a diagnostic sweep of the tissue priorto the ablation sweep.

The preferred method to collect noise-reduced gap data may also includeusing multiple gap distances to approximate incident angle of beam ontissue S130. Step S130 is preferably performed after Step S120. Theprocessor preferably stores gap data taken continuously or periodicallyduring the ablation process or alternatively information stored during adiagnostic sweep prior to the ablation sweep. Preferably, threeconsecutive or closely spaced gap distances are used to calculate theangle of incidence of the energy delivery system. Alternatively, anysuitable number of points may be used. The angle of incidence may beused in further steps to improve the interaction of the ultrasoundenergy beam with the tissue during the ablation process and, ultimately,create a desired transmural lesion of the tissue.

The preferred method to collect noise-reduced gap data may also includeadjusting the settings of the energy delivery system based on gap dataS140. Step S140 is preferably performed after Step S130. Step S140functions to adjust the ablation process to account for fluctuations ingap distance and/or angle and may function to prevent damage to thetissue or the device. Based on the gap distance and/or beam incidenceangle, the processor preferably controls the energy beam emitted fromthe energy source by modifying the electrical signal sent to the energysource via the electrical attachment such as the frequency, the voltage,the duty cycle, the length of the pulse, dwell time, and/or any othersuitable parameter. The processor may alternatively reposition theenergy delivery device.

Pre-mapping method. As shown in FIG. 12, a preferred method ofpre-mapping includes collecting data over the surface of the tissue ofinterest S200, generating a tissue map S210, and using the tissue map toexecute the ablation process S220. The pre-mapping method, which ispreferably performed as part of a diagnostic sweep prior to the ablationprocess, functions to create an anatomical map of the tissue to be usedduring the ablation process. The pre-mapping method may further functionto identify anatomical elements such as the pulmonary vein.

Step S200, which recites collecting data of the surface of the tissue ofinterest, functions to move the energy delivery system systematicallyover the tissue, periodically collecting gap distance data. The energydelivery system is preferably moved over the tissue in a horizontalzig-zag pattern, but the energy delivery system may alternatively bemoved over the surface in any suitable pattern that sufficientlycaptures data for the tissue of interest. The path is preferably asingular continuous path but may alternatively include multiplediscontinuous paths to capture tissue features from varying angles. Asthe energy delivery system moves above the surface of the tissue, thesensor preferably collects gap data using a method similar to thatdescribed above.

Step S210, which recites generating a tissue map, functions to generatea computer model of the tissue surface. The computer model is preferablygenerated using the gap data collected during Step S200 by associatingthe gap distance with the position of the energy delivery system duringthe data collection. The model preferably provides relative distanceinformation for the surface. The model may additionally and/oralternatively may be used to interpolate the surface angles andcontours. The computer model is preferably represented as a 2D image (totake advantage of image processing techniques), a 3D point cloud, 3Dsurface, or any other suitable format.

Step S220, which recites using the tissue map to execute the ablationprocess, functions to predict ablation paths and energy delivery systemsettings. Step S220 preferably includes the sub-steps finding anatomyfeatures S222, planning ablation path distances S224, and using angleinformation to adjust energy settings S226.

SubStep S222, which recites finding anatomy features, functions toidentify anatomical structures to obtain the orientation of the energydelivery system within the heart cavity. Preferably, the pulmonary veinis identified as an area that defines a recess in the surface of themodel. Alternatively, other anatomical features may be identified bysize, shape, or any other suitable characteristic from the tissue map.

SubStep S224, which recites planning ablation path distances, functionsto create a route with optimized gap distances that the energy deliverysystem will move through. The ablation path preferably has circulargeometry, but may alternatively be elliptical, polygonal, or any othersuitable shape. The gap distances from the tissue for each position areset to an optimal distance for the ablation process, preferably between2-25 mm, more preferably between 2-20 mm, and even more preferablybetween 2-15 mm.

SubStep 226, which recites using angle information to adjust energysettings, functions to optimize the beam energy for proper transmurallesions. The processor preferably makes appropriate changes to thefrequency, voltage, duty cycle, power, and/or dwell time of the energydelivery system.

Method for Detecting Wall Motion. As shown in FIG. 13, the preferredmethod for detecting wall motion includes scanning a portion of tissuerepeatedly over a short time duration S300, calculating the motioninformation for the tissue S310, and accounting for tissue motion S320.The preferred method provides information to more accurately determineif and when the device has achieved transmurality, which may reduce thepotential damage of the tissue, insufficient ablation of the tissue, ordamage to the device.

Step S300, which recites scanning a portion of tissue repeatedly over ashort time duration, functions to collect sample gap data (the data setof the separation between the tissue and the energy delivery system)during the periodic motion of the heart tissue. Preferably, the sensorinterrogates a singular spot of the tissue (or a group of closely spacedpoints) multiple times during a brief period of time (preferably overthe time span of 5 ms or less, based on the necessary Nyquist samplingfrequency for a heart rate of 100 beats per minute. The gap dataobtained during the interrogation for a static system will generallyremain constant, but—in the case of moving tissue—the gap data willgenerally vary over time. Alternatively, the gap data may be collectedduring a normal (preferably slow moving) diagnostic scan, where asingular spot is not repeatedly interrogated. In this alternative,closely spaced points are approximated as a single point, and the motionis approximated over the area defined by these points.

Step S310, which recites calculating motion information for the tissue,functions to generate the variance, velocity, and/or acceleration valuesfor the tissue to be used in Step S320. The processor preferablycalculates the variance of an A-mode data set, but the processor maycalculate the variance on any suitable data. The variance preferablycorresponds to the amplitude of the periodic displacement of the tissue.The motion variance is preferably used to position the energy deliverysystem 10, and set the parameters of the energy beam used for ablation.

Step S320, which recites accounting for tissue motion, functions toalter the position or energy beam settings of the energy deliverysystem. Step S320 may further function to ensure that a transmuraltissue lesion is created during ablation. Preferably, the processor usesthe variance of motion calculated in Step S310. The variance ispreferably used to position the energy delivery system such that the gapdistance is maintained within a suitable gap distance range. The gapdistance is preferably maintained preferably between 2 and 25 mm, morepreferably between 2 and 20 mm, and even more preferably between 2 and15 mm. Alternatively, the energy beam may be set to accommodate for thevariance in position.

In an alternative embodiment, Steps S310 and S320 may use the velocityand/or acceleration to predict motion. In this alternative, Step S310preferably includes the calculation of velocity, acceleration,frequency, and/or any other property of the tissue motion. The processorpreferably uses the calculated motion parameters and the periodic motionof the heart to predict the gap distance at any given time. In Step 320,knowledge of the exact gap distance is then used to reposition andchange the energy beam settings for near optimal tissue ablation.

As an additional step, the method may include identifying tissue typebased on tissue motion S330. This step functions to identify sensitivetissue (tissue not to be ablated) or anatomical structures to use as areferential positioning. The processor preferably compares the recordedmotion of the tissue to an anatomical model of tissue. The model may bean average variance of motion or may be more fully defined includingmodeling of surrounding tissue. The tissue velocity, magnitude ofmotion, motion frequency, or any other suitable characteristic obtainedfrom the motion profile can be used to distinguish different types oftissue. As an example of tissue identification, the atrial wall tends tomove much more than the connective tissues around the heart. Theprocessor may distinguish the two types of tissue by the magnitude ofmotion variance.

As an additional step, the method may further include detecting tissuethickness S340. Step S340 functions to prevent over and under ablationof tissue. The variance of tissue motion corresponds indirectly withtissue thickness (greater variance corresponds to thinner tissue and, incontrast, small variance corresponds to greater thickness). Preferably,the tissue thickness is based on the variance of tissue motion.Alternatively, the mechanical properties of average tissue such asstiffness are known and kinematic models of the tissue can be used forcomparison and identification of tissue.

Method of Monitoring Lesion Depth. As shown in FIG. 14, the method ofmonitoring lesion depth during ablation includes obtaining a standardlesion ratio S400, sensing initial tissue backscatter signal beforeablation S410, ablating tissue S420, sensing tissue backscatter signalof ablated tissue S430, comparing current lesion ratio to standardlesion ratio S440, and ceasing ablation when the current lesion ratioand the standard lesion ratio are equal S450. Monitoring the lesiondepth during ablation functions to form a desired transmural lesion andprevents over-ablation of the tissue.

Step S400, which recites obtaining a standard lesion ratio, functions togenerate a numerical value (the standard lesion ratio) that isassociated with a transmural lesion. The standard lesion ratio furtherfunctions as a value to which other lesion ratios can be compared toassess if transmurality has been reached. Preferably, the standardlesion ratio is a normalized backscatter signal value of a transmurallesion. Alternatively, the standard lesion ratio may be a normalizedsignal pattern, a frequency, or any other signal property that is uniquefor a transmural lesion. Step S400 is preferably performed with twosub-steps including sensing a tissue backscatter signal with highechodensity S402, and sensing a tissue backscatter signal with lowechodensity S404. The ratio of the tissue backscatter signal with highechodensity and the tissue backscatter signal with low echodensitypreferably make up the standard lesion ratio. Alternatively, thestandard lesion ratio may be a laboratory-determined value, which ispreprogrammed into the processor, or any other suitable value.

Step S402, which recites sensing a tissue backscatter signal with highechodensity, functions to obtain a signal sample of tissue where atransmural lesion is present. Preferably, the sensor detects anultrasound reflection from tissue with a transmural lesion. Morepreferably, the lesion results in increased backscatter and anattenuation of ultrasound. The amount of backscatter and attenuationpreferably distinguish the tissue backscatter signal of ablated tissuefrom tissue that has not undergone ablation.

Step S404, which recites sensing a tissue backscatter signal with lowechodensity, functions to obtain a signal sample of tissue that has notundergone ablation. Preferably, the sensor detects an ultrasoundreflection from tissue that has not undergone ablation, but is ofsimilar thickness to the high echodensity sample. Tissue of similarthickness may be obtained by scanning tissue with close proximity tothat of the ablated tissue sample, tissue with similar wall motion, oridentical location (but measuring the low echodensity signal beforeablation occurs) or any other suitable combination of locations withsimilar tissue thicknesses. Alternatively, the sensor may detect anultrasound reflection from a portion of tissue that was occluded (in ashadow) during ablation. In most situations, the tissue without a lesionhas less backscatter and less attenuation of ultrasound than tissue witha lesion.

Step S410, which recites sensing initial tissue backscatter signalbefore ablation, functions to obtain the tissue backscatter signal withlow echodensity for the current lesion ratio. Preferably, the signal issensed right before ablation begins for a portion of tissue.Alternatively, the initial tissue backscatter signal may be obtained forall points during a diagnostic sweep of the tissue. As anotheralternative, the initial tissue backscatter signal may be sampledrepeatedly during the ablation by sensing the tissue backscatter signalfrom tissue of close proximity, tissue occluded during ablation (in thebeam shadow), or any other suitable location.

Step S420, which recites ablating tissue, functions to increase thelesion depth of the tissue by ablating the tissue in an incrementalamount. Step S420 is preferably repeated several times during the courseof the method. In a first variation, the ablation steps (i.e., the depthof ablation during one cycle of Step S410) incrementally add to approachtransmurality of the tissue, moving from gross ablation steps to smallablation steps. During the first iteration of Step S420, the ablation ofthe tissue is such that transmurality is not expected, but the ablationstep is large enough to ablate a significant portion of the tissuewithout over ablating the tissue. In further iterations, the ablationpreferably approaches the state of transmurality in an approximatelyasymptotic manner; each step is a smaller ablation depth. The finaliteration preferably achieves transmurality. Additionally, estimation oftissue thickness made by the sensor or from an outside source may beused to more efficiently determine ablation steps. In a secondvariation, each ablation step may be identical in size regardless ofiteration or thickness estimation. As another alternative, ablation mayoccur continuously if Step S430 and Step S440 also occur continuously orperiodically during the process.

Step S430, which recites sensing tissue backscatter signal of ablatedtissue, functions to obtain the tissue backscatter signal with highechodensity for the current lesion ratio. The sensing of the tissuebackscatter signal preferably occurs after each ablation step iscompleted and preferably occurs periodically or continuously. Thesensing of the tissue backscatter signal may, however, occur at anyother appropriate time.

Step S440, which recites comparing current lesion ratio to standardlesion ratio, functions to assess if transmurality has been reached.Preferably, the current lesion ratio is based on the initial tissuebackscatter signal obtained in Step S410 and the tissue backscattersignal during ablation of Step S430. The current lesion ratio is thencompared to the standard lesion ratio obtained in Step S400. If thecurrent lesion ratio is less than the standard lesion ratio (i.e.,transmurality has not been reached), ablation preferably continues andSteps S420, S430, and S440 are preferably repeated. If the values areequal (i.e., transmurality has been reached or exceeded), the processproceeds to Step S450. Alternatively, any suitable means of comparingthe ratios may be used, including comparisons that do not rely uponactual ratios but rather other numerical values.

Step S450, which recites ceasing ablation when the current lesion ratioand the standard lesion ratio are equal (or within a predeterminedthreshold of equity), functions to end the ablation process for thetissue. After Step 450, the energy delivery system preferably moves tothe next section of tissue to be ablated.

As a person skilled in the art will recognize from the previous detaileddescription and from the figures and claims, modifications and changescan be made to the preferred embodiments of the invention withoutdeparting from the scope of this invention defined in the followingclaims.

What is claimed is:
 1. A method for ablating tissue, said methodcomprising: providing an elongate shaft having an ultrasound transducerand a sensor adjacent a distal portion of the elongate shaft, theultrasound transducer providing a collimated beam of ultrasound energy;positioning the ultrasound transducer adjacent target tissue fortreatment; sensing the target tissue with the sensor thereby determiningcharacteristics of the target tissue; ablating the target tissue withthe collimated beam of ultrasound energy thereby forming a continuouslesion in the target tissue; and adjusting the ablating based on thedetermined characteristics of the target tissue, and wherein the sensoris a single ultrasound transducer and the ultrasound transducerproviding the collimated beam of energy is the single ultrasoundtransducer.
 2. The method of claim 1, wherein the sensing is conductedbefore the ablating.
 3. The method of claim 1, wherein the sensing isconducted while the ablating is performed.
 4. The method of claim 1,wherein the adjusting comprises adjusting a power of the beam ofultrasound.
 5. The method of claim 1, wherein the adjusting comprisesrepositioning the ultrasound transducer relative to the target tissue.6. The method of claim 5, wherein the repositioning comprises rotatingor translating the ultrasound transducer relative to the target tissue.7. The method of claim 1, wherein the sensing comprises determining adistance between the ultrasound transducer and the target tissue.
 8. Themethod of claim 1, wherein the sensing comprises determining a thicknessof the target tissue.
 9. The method of claim 1, wherein the sensingcomprises determining properties of the target tissue after ablationthereof.
 10. The method of claim 1, wherein the sensing comprisesdetermining a depth of at least a portion of the continuous lesion. 11.The method of claim 1, wherein the sensing comprises determining aformation of at least a portion of the continuous lesion.
 12. The methodof claim 1, wherein the sensor further comprises a coating layer. 13.The method of claim 1, wherein sensing the target tissue comprisessensing the beam of ultrasound energy provided by the ultrasoundtransducer and reflected back from the target tissue.